Rename column/index fields of DataColumnSideCar

[asn-d6]

Working on #3745 had me confused on why sidecar.column is a list of cells, instead of the column index. This makes these fields a bit more clear.

It's an opinionated PR so feel free to NACK.

[ppopth]

When I see column, I literally think of a column of a table, not an index. Like when you think of a row, you think of a row that has cells, right? not an index because an index doesn't have cells.

I think index also already makes sense, because it's an index in a column sidecar, so it's an index of a column.

[asn-d6]

Closing due to lack of interest